The invention relates to a double-acting piston-pump for cryogenic mediums. The pump has a piston disposed in a vertical cylinder having a base and a cover. The base and cover each are provided with an inlet and an outlet valve. A drive rod for the piston extends through either the cover or base of the cylinder. The pump is suitable for both liquid and gaseous cryogenic mediums such as helium, nitrogen, hydrogen and the like. Practically no frictional forces are produced by the pump so that the pumped cooling liquid can therefore not evaporate.
Cooling circuits for conducting a cryogenic fluid such as liquid helium or liquid nitrogen are required for cooling electrical apparatus having superconductors such as a super conducting cable and the like. Pumps are provided to circulate such cryogenic liquid and operate within a secondary circuit.
With such cooling circuits, there are two different forms of operation. In the first, the heat is carried away through evaporation of the cooling liquid. A two-phase mixture composed of liquid and of gas then circulates in the cooling circuit. In the second form of operation the cooling liquid is circulated in a single-phase liquid state under pressure with fixation of the boiling point by means of a heat-exchanger. The heat is carried away above the specific heat of the cooling liquid. The operational pressure in the circuit can be adjusted by means of the gas stored in a vessel. The pumping pressure of the pump serves only to overcome the flow losses in the cooling circuit.
When placing a cooling circuit of liquid helium into operation, first the circuit can be tanked with liquid from a cryostat by means of a cold valve. After an initial accumulation of gas, a two-phase mixture circulates so that by switching the cold-valve, it is possible to switch over the operation to a circuit under pressure having a fixed boiling point. This results in a further requirement for the characteristics of a pump for cryogenic mediums; namely, the use of a mixed circuit. With such a pump the time for filling the circuit can be kept small. Furthermore, such a cryopump must meet other requirements: the throughput of cooling liquid must be constant and be variable over a great range. The pump should also be able to pump cold as well as warm gas.
For pumping liquid helium in liquid nitrogen, some special requirements also have to be met in the constructional configuration of the pump, because, at very low temperatures, the various materials undergo great and different contractions. At the temperature of liquid helium for example metals shrink up to about 0.4% and synthetic materials, used as sealing materials, shrink as much as 2.4%. Furthermore, the elasticity of the materials also alters. For example, rubber and synthetic substances become hard and brittle. It is generally not possible to use lubricants, because of the danger of contaminating the pumped cryogenic liquids, and because they become solid at low temperatures. Disturbances through congealing gas in the internal spaces of the pump during cool-down are to be avoided. Helium tends to oscillate in pipes, and should not be excited into such oscillations by the pump.
In order to provide a cooling circuit having increased pressure, care must also be taken that all parts of the pump are constructed for such an operating pressure. This requires in particular adequate sealing of the pump housing against the corresponding over-pressure. On the one hand, the passages through the cylinder wall for the drive rod for the piston must be adequately sealed; and on the other hand, such a sealing of the drive rod against the operative pressure produces an unallowable increase of frictional losses during the motion of the piston. This pressure-tight passage therefore forms an unallowable source of heat for the entire arrangement.
Rotary pumps are not suitable for pumping helium, because of the low density of helium, and because such pumps are able to produce only a small pressure difference in correspondence to the relative density. Bucket-wheel pumps, gearwheel pumps, and Roots pumps which develop pressure while rotating are indeed capable of producing a substantially higher pressure difference. However, their use depends in particular on the configurations possible relating to heat-technology with respect to production of gas, or of gas losses, at the various rotary speeds. Furthermore, such pumps have the usual mechanical elements such as passages for rods and stuffing boxes which create considerable operational difficulties. Such pumps are moveover generally not suitable for pumping gas.
In contrast, piston pumps and membrane pumps have the advantage that their volumetric throughput is variable, and that great pumping heads are possible with small throughputs. They are also capable of pumping cold and warm gases. There are also many construction possibilities available in selecting the drive for the pistons. It is for example possible to make use of a crank-drive, of a solenoid on the outside, or even of a super-conductive coil-drive inside the cryogenic vessel.
In the case of a membrane-pump system, the quality of the spring-bellows is of primary importance with reference to its life and the maximum attainable number of strokes per minute. Tombac is a suitable spring-bellows material; and the spring constant at the temperature of liquid helium is also less than at room temperature. The life of such tombac undulated members is, however, at 10.sup.5 to 10.sup.6 strokes, relatively short. In a pump for cryogenic liquids known from the journal Cryogenics April 1965, pages 107 to 109, the undulated member is therefore made of nickel. There is provided for the drive of the known pump, a superconductive alternating-current magnet that brings a niobium ring into oscillation. Through use of the superconductor, this pump is however limited to pumping helium. Furthermore, the undulated member permits no great pressure, and pumps only a limited quantity, because with such pumps, the stoke is limited to a few percent of the length of the undulated member, so that the elastic limit of the material of the undulated member may not be exceeded.
The requirements for a pump for cryogenic mediums, as described at the outlet, are therefore best met by a piston-pump immersed in the cooling liquid. The journal Review of Scientific Instruments, volume 41, No. 10, pages 570 to 573, describes a piston-pump whose cylinder consists of nonmagnetic material. The piston is carried along by two ring-magnets situated outside the cylinder and, guided by a guide-shaft, and driven by a drive-rod. This cylinder material however has only poor heat conduction, and through the normally-conductive magnets, a relatively great inflow of heat is obtained. It is therefore unsuitable for helium.
In the journal Advances in Cryogenic Engineering, volume 11, 1965, pages 530 to 535, a pump is described that works on the principle of a double-piston arrangement, which indeed makes possible a constant pumping; however, the removal of the heat is poor because of the use of high-grade steel, and its dead volume is realtively great. Consequently the quantity pumped in operation diminishes. The filling in the outer space of the cylinder evaporates, and thus the pumping becomes irregular.
U.S. Pat. No. 3,456,595 discloses a pump for cryogenic liquids which has a piston that moves in a vertical cylinder. A drive-rod for this piston passes through the cover of the cylinder. The cover and base each contain one induction valve and one outlet valve. The cylinder is first filled with a gas-liquid mixture. As the piston moves, first an excess of liquid flows through openings in the cylinder wall, which act as auxiliary valves, and returns to the liquid tank. Then the liquid-gas mixture becomes compressed, through which the gas becomes liquified again. Only then do the outlet valves open. Because of the openings in the cylinder wall it is not possible to operate with over-pressure. A closed circuit for the pumped medium is also not possible.
It has now been found that a pump that is to meet all the foregoing requirements must be constructed in such a way that the pumped cryogenic liquid shall not evaporate during the pumping. Consequently it is necessary to prevent practically any introduction of heat from the outside, and also, above all, to prevent any unallowable generation of heat by the pump itself. These requirements can only be met when only a negligible quantity of frictional heat is introduced at the passage for the drive rod and, if at the same time, any considerable flow losses in the inlet and outlet conduits for the cooling medium directly at the punp cylinder is prevented, particularly in the inlet and outflow valves.